Examples of the analytical, method using gas-liquid extraction operation in analysis by a GC or GCMS include a purge-and-trap method (hereinafter referred to as “P & T” in some cases) and a head space method (hereinafter referred to as “HS” in some cases). These analysis methods are described in the water supply law under the authority of the Ministry of Health, Labor and Welfare.
(1) P & T Method and Problems Thereof
In P & T method, a gas having a small solubility in a solution is flowed so that an analyte dissolved in the solution is forced to be extracted into the gas phase. The extracted analyte is temporarily retained on an adsorbent for condensation of the analyte. Subsequently, the adsorbent is rapidly heated for desorption of the heated analyte from the adsorbent. The analyte is then introduced to the GC analytical column.
In extraction of an analyte from water by P & T method, the purging efficiency R in the ideal state can be obtained by Equation 1.
                    R        =                                            m              a                                      m              0                                =                      1            -                          exp              ⁡                              [                                                                            -                      F                                        ·                    t                                                                              K                      ·                                              V                        L                                                              +                                          V                      G                                                                      ]                                                                        (                  Equation          ⁢                                          ⁢          1                )            
In Equation 1, F represents the purge gas flow rate, t represents the purge time, K represents the partition coefficient, VL represents the volume of a sample, VG represents the volume of gas phase in a vessel, m0 represents the amount of analyte in an original solution, and ma represents the amount of analyte out in the gas phase.
The larger the partition coefficient is, the higher total purge flow rate is required to increase the recovery rate. In the subsequent step of retaining the extracted analyte in the purge gas on an adsorbent, it is required to consider the total flow rate of the purge gas, since the adsorbent for use has a break-through volume for each analyte.
From Equation 1, it can be understood that the purging efficiency is increased by increasing the purge gas flow rate, reducing the partition coefficient K, reducing the volume of a sample, and reducing the gas phase portion in a purge vessel.
Problems of P & T Method
1) Recovery Rate of Water-Soluble Component Having a Large Partition Coefficient
In P & T method, even with an increased total purge flow rate, non polar components with a small partition coefficient coexisting in water are mainly recovered, so that it is difficult to improve the recovery rate of a water-soluble component having a large partition coefficient K.
It is said that a musty odor component in tap water has a threshold of 1 ppt. In other words, a man recognizes musty odor of an analyte even when it exist in an amount of 1 pg in 1 ml of tap water. It is said that trichloroanisole formed from phenols and the residue due to sterilization with hypochlorous acid has an olfactory threshold of 0.03 ppt. On the other hand, measurement sensitivity of GC/MS cannot reach the level of human sensitivity. Though depending on the amount of contaminants not to be measured or the like, several pg of analyte is required to be introduced to a separation column of GC so as to reach the mass analysis part for the detection of analyte.
In the measurement methods including P & T method and HS method, use of 20 ml of sample water is recommended in analysis (corresponding to 20 pg for a musty odor component concentration of 1 ppt).
At a sample extraction temperature of 40° C., although 100% of benzene can be recovered with a total purge gas flow rate of 800 ml, only 21.8% of 2-MIB and 21.4% of Geosmin can be recovered. Further, an increased total purge gas flow rate for the purpose of improving the recovery rate of an analyte having a large partition coefficient without consideration of the breakthrough volume of an adsorbent causes breakthrough of the analyte. In other words, there exists the upper limit for the total purge gas flow rate depending on the adsorbent for use (Table 1).
TABLE 1Extraction efficiency with an amount of the sample of 20 mLand a vessel having a space volume of 20 mLPurge flow rate of 20 ml/minTotal purgeTemperaturePartitionflow rate (ml)(° C.)coefficient K4008001600Benzene403.1899.2%100.0%100.0%502.6399.6%100.0%100.0%602.2099.8%100.0%100.0%MB40161.5011.6%21.8%38.9%50101.6217.7%32.3%54.1%6065.7525.9%45.1%69.8%Geosmin40164.7511.4%21.4%38.3%5096.1218.6%33.8%56.1%6057.9328.8%49.3%74.3%
2) Problems Caused by Increased Purge Flow Rate
The purge vessel of conventional P & T method causes scattering of liquid droplets as the purge flow rate is increased. In real samples, metal, rust, microbes, bacteria or the like deposit on the piping, causing a trouble in the apparatus. It is also said that bubbling of the sample by purging reduces the gas-liquid extraction efficiency. With a slow purge flow rate, the time for analysis is prolonged to increase the total purge gas amount (refer to Anal. Chem., 58 (1986) 1822, James F. Pankow).
3) Slow Mass Transfer of Analyte in Water to Gas Phase
It is considered that the reasons for needing time for gas-liquid equilibrium in P & T method include: a) homogenization of gas phase concentration in a bubble, b) mass transfer at the gas-liquid (bubble) interface, and c) mass transfer in water. The largest contribution comes from c).
It is therefore necessary to improve the contact efficiency between gas and liquid by making many small bubbles in a purge vessel so as to uniformly flow the bubbles in the purge vessel and increasing the gas-liquid contact area.
Installation of a sintered filter with a fine mesh, which is called frit, in the purge vessel is thus important. In real samples, however, fine particulate substances, algae or the like accumulated on the frit cause contamination of the samples, resulting in variation in the recovery rate and necessity of frequent maintenance of the frit or the like.
4) Cryofocus
Since P & T method has a poor extraction efficiency, cryofocusing with liquid nitrogen is employed to introduce all the amount of extracted analyte to a GC or GC/MS. In performing continuous automatic analysis for a 24-hour period in a water purification plant, work for supplying liquid nitrogen is required. And it is difficult to be fully automated, and resulting in disadvantage in analysis cost.
(2) HS Method and Problems Thereof
In HS method, a sample is enclosed in a hermetically sealed vial, heated at a constant temperature for a predetermined time, and kept warm for reaching a gas-liquid equilibrium state. Subsequently, a predetermined amount of gas phase portion is sampled and analyze.
Problems of HS Method
The concentration in gas phase by HS method can be obtained by Equations 2.Concentration in gas phase CG=CL0/(K+β)Partition coefficient K=CL/CG Phase ratio β=VG/VL  (Equations 2)
In the equations, CG represents the concentration (g/cm3) of the analyte in gas phase, CL represents the concentration (g/cm3) of the analyte in liquid phase, VG represents the volume of gas phase, VL represents the volume of liquid phase, and CL0 represents the concentration of a sample before reaching equilibrium.
Based on Equations 2, in the case of a water-soluble component having a large partition coefficient K, an analyte to be extracted in the gas phase has a poor recovery rate. The partition coefficient K needs to be reduced to increase CG. It is understood that raising the vial temperature, performing salting out operation (for a water sample), and addition of an acid or base are effective.
Meanwhile, reduction in β (increase in the amount of a sample or reduction of gas phase part) is not very effective in practice to improve the sensitivity.
Recently, a technique has been developed to increase the sensitivity of HS method, in which a portion of gas phase in gas-liquid equilibrium state is sampled multiple times from a sample vial and analyte is an adsorption tube and then desorbed by rapid heating for introduction to an analytical column (multiple HS method). The improvement of sensitivity for an analyte having a large partition coefficient K can be expected by the plural condensation in the multiple HS method. In addition to taking time in reaching gas-liquid equilibrium, however, the time required for sample pretreatment operation is increased due to the multiple sampling in the gas-liquid equilibrium state.
In the multiple HS method, the head space gas after reaching gas-liquid equilibrium is repeatedly condensed and collected at an adsorbent multiple times. In P & T method, gas-liquid equilibrium is made with microbubbles for continuous extraction. Although there is a difference in that HS method involves batch treatment and P & T method involves continuous extraction, HS method and P & T method are the same in principle, so that the amount of extraction depends on the partition coefficient.
The major problem of both is that although the gas concentration of analyte is high immediately after starting the extraction operation after introduction of a sample to a vessel or a purge vessel, since the partition coefficient (ratio of the concentration in solution to the concentration in gas) is constant, the concentration in the gas is reduced as the concentration of solution is reduced due to the extraction of the analyte from the solution. In other words, in the last half of extraction operation in P & T method, the amount of extraction of analyte is reduced in spite of consuming a large amount of extraction gas.
Gas-liquid countercurrent contact extraction is a gas-liquid extraction operation having potential to solve the problem of the conventional methods. The method is employed for removal or recovery of a predetermined component from a large amount of liquid in the chemical engineering field like plants or the like (refer to Japanese Patent Laid-Open No. 10-57947 and Japanese Patent No. 3006894).
In Japanese Patent Laid-Open No. Heisei 10-57947, a method for separating a large amount of ammonia is disclosed, in which an ammonia-containing solution is splashed into gas phase to be micronized for extreme enlargement of the gas-liquid contact area, so that the dispersion is performed with an enhanced emission efficiency of ammonia into gas phase. Accordingly, this method cannot be suitable for a method and apparatus for analyzing a small amount or trace of a sample.
In Japanese Patent No. 3006894, a structure with an erected pipe line bent in the vertical direction for flowing a liquid from above and a gas from beneath, and a structure with a pipe having an inner wall part with a wick for a liquid phase transfer by capillarity are disclosed.
When a pipe reactor (pipe line) indicated therein is tilted, the liquid does not contact with a filling material 6, resulting in insufficient gas-liquid contact surface area. Accordingly, a wick is arranged on the inner wall surface of a pipe, allowing for a liquid phase transfer of the liquid by capillarity. Although the liquid phase transfer by capillarity can be applied to a case having a large amount of liquid, the control of time is not effective in an analysis process for a trace of a liquid sample, and in the case that fine particles exist in the sample liquid, the fine particles deposited on a rotating member and the wick may cause contamination of the inner wall surface of the pipe. The liquid phase transfer by capillarity is, therefore, not suitable for continuous operation of the apparatus. Furthermore, in the case that a surfactant is contaminated in the liquid sample, or in the case that the liquid sample is a carbonated beverage, extraction of the sample itself may be difficult due to foaming in some cases, so that practical application cannot be achieved.
Literature “Analysis Sample Pretreatment Handbook” (by Masahiro Furuno, 2003, Maruzen Co., Ltd.) suggests the application of a gas-liquid contact extractor to GC analysis, flowing a sample liquid from above a helical pipe and sending a purge gas from beneath. However, the apparatus shown in the literature allows for reciprocal free flows of a sample liquid and purge gas only, without consideration of the control of the sample liquid and the purge gas and without consideration of the control of the relationship therebetween. Since the extraction efficiency between gas and liquid is extremely low due to the superficial mutual gas-liquid contact, so that it is difficult to use the extraction sample as an analysis sample. Practical application has not been achieved yet.